The long-term objectives are expansions of those pursued during the first term of this grant and are: a) to describe the spread of excitation and the associated potential distributions in the ventricles during spontaneous and paced beats, as affected by the rotating direction of ventricular fibers; and , b) to correlate the intracardiac activity measured in a given heart with the three-dimensional potential distributions produced by the same heart in a torso-shaped electrolytic tank and its surface. Health relatedness of the project: The purpose of clinical electrocardiography is to provide information on it is indispensable to acquire a better knowledge of the intracardiac electric sources than is available at present, and to improve our understanding of the correlations between cardiac sources and extracardiac potential fields. Experimental design: Intramural excitation and intramural potentials will be studied by inserting 100 to 150 multi-electrode needles into the ventricular walls and septum of isolated dog hearts. Fiber directions will be determined histologically in the entire heart. The respective roles of the Purkinje system, the rotational antisotropy of the heart muscle, the conductivity of the surrounding media and the presence of localized injuries in determining wavefront shapes and potential distributions will be assessed by using heart preparations that enable the various factors to be studied separately. The effects of the same factors on epicardial potentials will be studied using high resolution (3-4mm) electrode arrays that cover the entire ventricular surface. Measured data will be compared with numerical simulations. Hearts with 100 to 150 inserted needles will be immersed into a torso-shaped electrolytic tank and the intracardiac and extracardiac electrical activity will be measured during spontaneous and paced heart beats. In two other groups of experiments, a) endocardial potentials will be inversely computed from signals recorded from an intracavitary probe, and b) epicardial potentials will be computed from electrocardiograms recorded from the surface of the tank. Results of both computations will be compared with measured data. Primary significance of these studies lies in the measurement of both intramyocardial and extramyocardial potentials in the same preparation, thus permitting greater certainty in relating body surface, epicardial and endocardial events to their origins within the ventricular walls. As a consequence, improved understanding and assessment of normal and abnormal cardiac phenomena will be realized.